1. Field of the Invention
The present invention relates to the exercise of control over the voltage applied to a capacitive load, a light emitting capacitor for example, and particularly to the energization of electroluminescent lights. More specifically, this invention is directed to a drive circuit for an electroluminescent (EL) display and particularly to an EL lamp driver for generating an alternating output voltage which is variable in magnitude and frequency. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
Drive circuits for EL lamps are well known in the art. For examples of such prior art lamp drives, reference may be had to U.S. Pat. No. 5,066,895 and U.S. Pat. No. 4,845,489. For a discussion of the construction and utility of EL lamps, reference may be had to U.S. Pat. No. 5,066,895.
An EL lamp is a light-emitting device which appears to the circuit supplying energy thereto as a capacitive load. During the service life of an EL lamp, the capacitance thereof will decrease with age. As the capacitance decreases with age, the capacitive reactance will also vary thus causing the output frequency of the lamp driver to change. Within its operating range, the brightness of an EL lamp will vary directly with frequency and thus will also tend to increase with age. However, the color of the light generated by an EL lamp will also vary with frequency and for many applications a change in color is undesirable and/or unacceptable. Further, the current drain imposed on the drive circuit will also increase with frequency and for many operating environments, particularly where the power supply is a battery, such an increase in power consumption is unacceptable.
It is also to be noted that, if the frequency of the drive voltage applied to an EL lamp is constrained to be constant, the color will remain the same but the intensity of the light produced by such a lamp will diminish as the electrical characteristics of the lamp vary with age. Further, if drive voltage frequency is not held constant, the increased brightness incident to allowing the frequency of the drive voltage to increase as the capacitance decreases does not fully compensate for the reduction in total output intensity per unit area. The intensity of an EL lamp is a function of the voltage applied thereto.
The prior art EL lamp drivers and drive techniques did not offer a total solution to the operational problems resulting from the above-discussed change in lamp characteristics with age. Particularly, the prior art did not permit the independent adjustment of the output voltage and output frequency of an EL lamp driver. Also, prior EL lamp drivers did not provide an output voltage and output frequency which were independent of the load, i.e., the physical size of the lamp. A further significant deficiency of the prior art resided in an inability to exercise control over the lamp driver so as to obtain a constant "brightness" over the life of an EL lamp. Yet another deficiency of prior art EL lamp drivers was their relatively large size which resulted from the use of a transformer.